1. Field of the Invention
The present invention relates to a method and apparatus for molding a polyurethane foam. The method and apparatus are suitable for reactive injection molding (RIM for short) of polyurethane foams in order to form articles such as automotive steering wheel, steering wheel pad, instrument panel, console box lid, glove box lid, headrest, armrest, and air spoiler.
2. Description of the Prior Art
Polyurethane foams are usually produced by injecting a liquid polyurethane material, composed of a polyol component, an isocyanate component, and a blowing agent, into the mold cavity, permitting the polyurethane material to expand, thereby permitting it to flow and fill the mold cavity, and removing the polyurethane foam from the mold after curing. Among the known blowing agents are fluorochlorohydrocarbons (such as "Freon", a trade name of DuPont), methylene chloride, and water. An adequate blowing agent is selected according to the use and type of the polyurethane foam to be produced. Freon and methylene chloride (which are low-boiling solvents) expand the polyurethane material when they are vaporized by heat generated by the reaction of the polyol and isocyanate components. Water expands the polyurethane material by evolving carbon dioxide gas upon reaction with the isocyanate component.
Heretofore, low-boiling solvents (such as Freon and methylene chloride) have been the only blowing agent that can be used for the molding of polyurethane foams to form articles such as automotive steering wheels and steering wheel pads, which are composed of a surface skin layer of high density (without expansion or with a low expansion ratio) and an internal core of low density (with a high expansion ratio). There are two reasons for this.
(1) Low-boiling solvents (such as Freon and methylene chloride) permit one to control the amount of foaming due to vaporization by the use of a temperature gradient and a pressure gradient in the mold cavity. This produces the desired skin layer and the core part with ease. Thus, the resulting polyurethane foam has a good appearance and soft feel.
(2) Water has several disadvantages as a blowing agent. When used in a small amount (say, 0.1-1.0 part by weight for 100 parts by weight of polyol component), water produces only a small amount of carbon dioxide gas, giving rise to a foam having a high-density core. This necessitates using a larger amount of polyurethane material than necessary. Conversely, when used in a large amount (say, 1.0-2.0 parts by weight for 100 parts by weight of polyol component), water produces a sufficient amount of carbon dioxide gas, giving rise to a foam having a low-density core as well as a highly expanded skin layer which looks poor. Moreover, the carbon dioxide gas forms a large number of urea bonds which make the core hard and brittle and hence make the foam, as a whole, feel rigid. This problem associated with rigid feeling can be solved by selecting a proper composition for the polyol component and isocyanate component. However, this is not practicable because of the high material cost.
As mentioned above, low-boiling solvents (such as Freon and methylene chloride) have many advantages and are used as essential blowing agents for the molding of polyurethane foams composed of a skin layer and a core part.
Despite their advantages, Freon and methylene chloride have recently met with a worldwide campaign against their use. It is said that Freon, once released, diffuses into the atmosphere and destroys the Stratospheric ozone layer, resulting in an increase of ultraviolet rays reaching the earth's surface. Methylene chloride is considered to be toxic and carcinogenic.